The implantable cardioverter-defibrillator (ICD) represents an important innovation in the treatment of sudden cardiac death, but significant questions remain unsolved. Currently, left ventricular ejection fraction (EF) is the primary parameter used to select patients for ICD therapy. Impaired EF is shown to be a marker of increased cardiovascular mortality and sudden cardiac death, but has relatively low sensitivity to detect arrhythmic risk. There is emerging awareness of the limitations in using EF as the main risk stratification tool for ICD therapy. Sudden cardiac arrest accounts for a smaller proportion of deaths in patients with lowest EF than in patients with relatively preserved ventricular function. A variety of diagnostic tests have been proposed to improve the accuracy for selection of patients who need ICD therapy. Currently available data, however, do not support additional risk-stratification methods for selection of patients for ICD therapy.
The presence of myocardial scar forms the substrate for malignant arrhythmias. Heterogeneity in scar tissue create areas of slow conduction that generate the substrate for ventricular arrhythmia post-MI and in selected primary cardiomyopathies. Electrical dispersion, including both activation time and refractoriness, in heterogenic tissue is a known arrhythmogenic factor. Electrical abnormalities may lead to distorted myocardial function. Therefore, regional differences in electrical properties may cause heterogeneity of myocardial contraction and may be recognized as mechanical dispersion. Subtle contraction heterogeneity can be demonstrated by myocardial strain echocardiography. Myocardial strain echocardiography can accurately quantify timing and regional myocardial function. It has recently demonstrated heterogeneity of systolic contraction by echocardiography as mechanical dispersion, assumingly as a consequence of electrical dispersion in patients with long QT syndrome (LQTS) (Haugaa K H et al; Left ventricular mechanical dispersion by tissue Doppler imaging: a novel approach for identifying high-risk individuals with long QT syndrome. Eur Heart J 2009;30:330-7). LQTS is an inherited cardiac arrhythmic disease affecting children and young individuals with no visible structural alterations in the myocardium. Ion channel defects in LQTS result in dispersed electrical repolarization and it was showed that mechanical dispersion was present in these patients along with normal myocardial shortening. In these patients, mechanical dispersion was associated with ventricular arrhythmias.
The mechanism for electrical dispersion in post-MI patients and patients with primary cardiomyopathies, however, is different from LQTS patients. In post-MI patients and in patients with primary cardiomyopathies, delayed start of ventricular activation in scarred and heterogeneous myocardium leads to a dispersed recovery of excitability, resulting in dispersed electrical repolarization.
Hence, an improved method for estimating the risk for ventricular arrhythmias in a subject would be advantageous, and in particular a more efficient and/or reliable method for estimating the risk for ventricular arrhythmias in a subject who has previously suffered myocardial infarction would be advantageous.